The rest is lost in friction, vibrations, heat, and motion of the anvil.
In impact the stresses produced become very complex and difficult to measure, especially if the velocity is high, or the mass of the beam itself is large compared to that of the weight.
The difficulties attending the measurement of the stresses beyond the elastic limit are so great that commonly they are not reckoned.

Within the elastic limit the formulae for calculating the stresses are based on the assumption that the deflection is proportional to the stress in this case as in static tests.
A common method of making tests upon the resistance of wood to shock is to support a small beam at the ends and drop a heavy weight upon it in the middle.

(See Fig.

40.) The height of the weight is increased after each drop and records of the deflection taken until failure.

The total work done upon the specimen is equal to the area of the stress-strain diagram plus the effect of local inertia of the molecules at point of contact.
The stresses involved in impact are complicated by the fact that there are various ways in which the energy of the striking body may be spent: (_a_) It produces a local deformation of both bodies at the surface of contact, within or beyond the elastic limit.